In general, a plant including various types of machine tools transfers a material (a load), which are loaded in the plant, by using a servo motor, and at this time performance of a control system is greatly affected by load inertia according to a weight of the load, which is applied to the servo motor and a transfer shaft connected thereto.
Particularly, in a case in which a pitch of a transfer shaft ball screw is increased in order to reduce a cycle time and increase a transfer velocity, because an effect of reduction ratio by the ball screw is decreased, a change in load inertia becomes larger in accordance with a change in load weight, and thereby performance of the control system is greatly varied.
That is, not only bandwidth of the control system but also stability of the control system is affected by a relative ratio between inertia of the servo motor and load inertia seen from the corresponding servo motor. In general, as an inertia ratio of the load to the servo motor becomes small, the bandwidth of the control system becomes increased and stability of the control system becomes high, and in contrast, as the inertia ratio of the load to the servomotor becomes high, the bandwidth of the control system becomes decreased and stability of the control system deteriorates.
Further, since there are problems in that as the bandwidth of the control system becomes decreased, a delay between a transfer command and a following is increased because responsiveness to the command deteriorates, and as stability deteriorates, a following overshoot is increased, the problems, which are expected to occur need to be avoided by adjusting an appropriate control parameter, and therefore it is necessary to estimate the load inertia according to the load weight in real time.
To this end, “Inertia Estimating Controller and Control System (Japanese Patent Application Laid-Open No. 2010-148178)” of which an applicant is Fanuc Ltd. in Japan suggests a method in which if a sine torque command is transmitted to a transfer shaft servo motor in accordance with an inertia estimation start signal, load inertia is calculated by detecting an electric current and acceleration data of the servo motor when the servo motor vibrates by the torque command.
However, because the aforementioned method uses a method of vibrating the transfer shaft servo motor by generating the inertia estimation start signal every time a load is changed, and setting a parameter by measuring friction of a transfer shaft in advance, the method does not cope in real time with a change in friction property of the transfer shaft according to a load weight and an elapsed time, and as a result, there is a problem in that it is difficult to estimate the inertia or an estimation error becomes large in a case in which connection stiffness between the servo motor and the transfer shaft deteriorates due to a mechanical backlash.
In addition, “Method for Determining the Mass Moment of Inertia of an Electric Motor Drive System (U.S. Pat. No. 6,998,812)” of which an assignee is Dr. Johannes Heidenhain Gmbh in Germany suggests a method of estimating load inertia by transferring a material transfer shaft in all forward and reverse directions.
However, the aforementioned method degrades estimation performance of the load inertia due to a ripple of torque and acceleration data of a servo motor, which may occur by transfer shaft compliance at the time of turning the directions, and fails to reflect in real time a change in frictional force (that is, disturbance) even though frictional force of a transfer system in a constant velocity section and frictional force in an acceleration section are different from each other, and as a result, there is a problem in that calculation accuracy of the load inertia deteriorates.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.